PB Ch2, Modes of Reproduction, Crossing Techniques

⚑ PYQ: IFoS 2018 (Q2a, 10M) Distinguish between Seed production in self-pollinated and cross-pollinated crops (giving suitable examples).

2.3. Mechanisms Promoting Cross-Pollination

Nature has evolved diverse physiological and structural mechanisms to enforce cross-pollination, which prevents inbreeding depression in naturally outcrossing species.

1. Dicliny (Unisexuality) This occurs when plants produce flowers that have only one functional sex.

• Monoecious plants: Male and female flowers are located on the same plant (e.g., maize, castor, cucurbits, and coconut).
• Dioecious plants: Male and female flowers are located on entirely separate plants (e.g., papaya, spinach, asparagus, and date palm). Dioecy is the strongest mechanism for promoting outcrossing, as it enforces 100% cross-pollination.

2. Dichogamy (Temporal Separation) This mechanism involves the anthers and stigmas of a bisexual flower maturing at completely different times.

• Protandry: The anthers mature first, releasing pollen well before the stigma becomes receptive (e.g., maize, sunflower, carrot, and sugar beet).
• Protogyny: The stigma becomes receptive before the anthers dehisce and release pollen (e.g., bajra and cherimoya).

3. Herkogamy (Spatial Separation) The anthers and stigmas are physically positioned within the flower in a way that mechanically prevents self-pollination. Examples include the heterostyly found in Primula, the specialized tripping mechanism required in lucerne (alfalfa), and the complex floral structures of orchids.

4. Self-Incompatibility (SI) In this system, pollen from a plant fails to fertilize its own stigma despite being fully functional and viable.

• Gametophytic SI: Compatibility is determined by the single S-allele present within the haploid pollen grain (e.g., tobacco, lily, and clover).
• Sporophytic SI: Compatibility is determined by the diploid genotype of the mother plant that produced the pollen, involving dominance interactions at the S-locus (e.g., crucifers and Brassica species). This mechanism is heavily exploited in the commercial hybrid seed production of cabbage and cauliflower.

5. Male Sterility The plant produces non-viable pollen, but the female reproductive system remains fully functional. There are three main types: Genetic Male Sterility (GMS), Cytoplasmic Male Sterility (CMS), and Cytoplasmic-Genetic Male Sterility (CGMS). This trait is a cornerstone for creating commercial hybrids (further detailed in Chapter 9).

⚑ PYQ: IFoS 2021 (Q1b, 8M) Explain the mechanisms which promote cross-pollination.

2.4. Apomixis — Seeds Without Sex

Apomixis refers to the formation of seeds without the occurrence of fertilization. In this process, the embryo develops from an unreduced (diploid) cell within the ovule. Consequently, the offspring are genetically identical to the mother plant, meaning the seeds act as exact biological clones.

Types of Apomixis

• Recurrent apomixis: The embryo sac remains diploid, and the embryo develops without any fertilization event taking place.

  • Diplospory: The megaspore mother cell fails to undergo meiosis and directly forms an unreduced embryo sac (e.g., Taraxacum and Eragrostis).

  • Apospory: Somatic cells of the nucellus bypass the standard reproductive pathway to form an unreduced embryo sac (e.g., Panicum and Hieracium).
• Non-recurrent apomixis: A haploid embryo develops directly from an unfertilized egg cell. This is a rare occurrence, occasionally seen in maize haploids.
• Adventitious embryony (Nucellar embryony): The embryo develops directly from the somatic cells of the nucellus or the integument layers. This is highly common in horticultural crops like citrus and mango.
• Parthenogenesis: The embryo develops directly from an unfertilized egg cell, commonly found within broader apomictic plant systems.

Significance of Apomixis in Plant Breeding

• Fixing heterosis: Apomictic seeds perfectly replicate the maternal genotype, meaning the hybrid vigour (heterosis) of an F1 generation can be perpetuated indefinitely.
• Cost reduction: It eliminates the expensive and labour-intensive need for hybrid seed production during every single generation.
• Maintaining uniformity: It is highly useful for maintaining absolute rootstock uniformity in crops like citrus and mango.
• Line maintenance: Apomixis allows breeders to maintain inbred lines without the tedious process of selfing.
• Future breeding goals: Transferring apomictic traits to major staple cereals (like rice, maize, and wheat) holds revolutionary potential to permanently fix hybrid vigour for global food security.
• Limitations: Because apomixis bypasses genetic recombination, the resulting cloned populations suffer from reduced evolutionary adaptability over successive generations.

⚑ PYQ: IFoS 2023 (Q2bi, 5M) Differentiate between Apomixis and Parthenogenesis.

2.5. Crossing Techniques in Plant Breeding

Controlled crossing is the fundamental operation of any hybridization-based breeding programme. The specific techniques utilized will vary heavily depending on the floral biology of the target crop.

Step 1 — Selection of Parents The breeder must carefully choose parents that possess complementary characteristics (for instance, crossing a high-yielding variety with a highly disease-resistant variety). Both parents must demonstrate good General Combining Ability (GCA). Additionally, breeders should avoid selecting parents with similar genetic backgrounds, because a wider genetic distance typically yields superior heterosis.

Step 2 — Selection of Flowers The breeder selects healthy, well-developed flowers at the appropriate stage of bud development. This stage is usually just before anthesis (anther dehiscence), typically 1 to 2 days before the flower bud naturally opens. The optimum time for selection is early morning, as cooler temperatures support peak pollen viability.

Step 3 — Emasculation Emasculation is the physical or chemical removal of anthers from a bisexual flower before pollen dehiscence occurs, preventing any unintended self-pollination. The methods include:

• Hand/Forceps method: Fine forceps are used to meticulously remove the anthers. This is commonly used in wheat, rice, cotton, and brinjal.
• Cutting method: The corolla and anthers are simply cut away using sharp scissors, a technique frequently used for legumes.
• Suction method: Pollen is gently sucked out of the flower, a method reserved for very small flowers.
• Water flotation: Flowers are submerged in water, causing the pollen to either sink or wash away completely. This is utilized in rice and sorghum.
• Hot water emasculation: Flowers are submerged in water heated to 42–48°C for 5 to 10 minutes, which safely kills the pollen without harming the female organs. This is used in sorghum and rice.
• Alcohol emasculation: Flowers are briefly dipped in 57% ethanol for exactly 10 seconds. This is a common practice for lucerne.
• Chemical emasculation: Chemical Hybridizing Agents (CHAs) such as Maleic Hydrazide, Gibberellic Acid (GA), FW-450, RH-531, or ethrel are sprayed onto the plant to induce temporary male sterility. This is used in wheat, rice, and cotton.
• Genetic emasculation: Breeding programmes use Cytoplasmic-Genetic Male Sterility (CGMS) lines. Because the male is naturally sterile, no manual emasculation is needed, saving immense labour in maize, sorghum, sunflower, and hybrid rice production.

Step 4 — Bagging Immediately following emasculation, the flowers are covered with butter paper or polythene bags to protect the receptive stigma from stray, airborne pollen. These bags are carefully labelled with the specific cross combination, the date of emasculation, and the breeder's unique identification code.

Step 5 — Pollination Pollen is collected from the selected male parent at the peak time of anther dehiscence.

• Direct method: Freshly dehisced anthers are gently brushed directly onto the receptive stigma.
• Glass slide method: Pollen is collected onto a clean glass slide and then meticulously transferred to the stigma using a fine camel-hair brush.
• Petri dish method: Pollen is collected into a petri dish and applied to the stigma using a sterilized needle. Pollination is generally conducted in the early morning (between 8:00 AM and 10:00 AM) to align with optimum stigma receptivity.

Step 6 — Post-Pollination Care After pollination, the protective bag is re-fixed over the flower for an additional 2 to 3 days to prevent any late-stage contamination. A permanent tag is attached featuring the cross details, parent names, date of pollination, and the breeder's name. Once successful seed set is confirmed (usually after 5 to 7 days), the bags can be safely removed. Finally, the mature seeds are harvested and stored separately for future F1 evaluation.

2.6. Synchrony of Flowering & Planting Ratios

Synchrony of Flowering Synchrony of flowering refers to the precise matching of flowering times between the male (pollinator) parent line and the female (seed) parent line during hybrid seed production. This is absolutely critical to ensure that adequate pollen is available precisely when the female line's stigmas are receptive.

• Methods to achieve synchrony:

  • Staggered planting: The male parent is sown either earlier or later than the female parent, calculated based on the known differences in their maturation durations.

  • Environmental manipulation: Breeders can adjust irrigation schedules, fertilizer applications, or photoperiod exposure to artificially alter flowering times.
  • Hormonal application: Spraying Gibberellic Acid (GA3) can successfully delay or advance flowering by a margin of 5 to 7 days.
  • Multiple planting dates: The male parent is planted in successive batches on multiple dates to ensure a continuous supply of viable pollen throughout the entire flowering window of the female line.

Planting Ratios The planting ratio refers to the specific proportion and spatial arrangement of male rows to female rows within a hybrid seed production plot.

• Common ratios (Male : Female):

  • Maize: Commonly planted at a 2:4 ratio (though some breeders prefer 1:3 or 2:6 depending on the variety).

  • Sorghum: Standardly planted at a 2:6 ratio.
  • Bajra: Planted at a 2:6 ratio, utilizing a male-sterile A-line.
  • Hybrid rice: Planted at a 1:8 or 2:8 ratio. Because rice pollen does not travel well, supplementary pollination (like rope pulling) is crucial.
  • Sunflower: Typically planted at a 1:3 ratio.
  • Cotton: Due to the heavy reliance on manual hand-emasculation, cotton is planted at a 1:1 ratio.

⚑ PYQ: IFoS 2017 (Q3c, 15M) Distinguish between Synchrony in flowering and Planting ratio of parents.

★ CRITICAL FOR HYBRID SEED PRODUCTION: Synchrony ensures the correct timing of male and female flowering, while the planting ratio ensures the correct spatial arrangement of the plants. Both factors must be optimized in tandem to achieve high seed yields and maintain genetic purity. If synchrony is inadequate, the female line will suffer from low seed set. If the planting ratio is miscalculated, the plot will either experience a severe pollen shortage (if there are too few males) or an unacceptable loss in total commercial yield (if there are too many non-bearing male plants).